Place the butter into a large pot and melt over medium low heat. Add marshmallows and stir constantly until fully melted.
Remove from heat and add the cereal. Stir until fully combined. Add in M&Ms and mix quickly so the chocolate doesn't melt or colors bleed.
Press into a greased 9x11" baking pan. To make it easier to press down, butter your hands so they won't stick to the marshmallow or use parchment/wax paper to press down.